Check whether the string is a unix timestamp

I have a string and I need to find out whether it is a unix timestamp or not, how can I do that effectively?

I found this thread via Google, but it doesn't come up with a very solid answer, I'm afraid. (And yes, I cribbed the question from the original poster on the aforementioned thread).

Ok, after fiddling with this for some time, I withdraw the solution with date('U') and suggest to use this one instead:

function isValidTimeStamp($timestamp)
{
    return ((string) (int) $timestamp === $timestamp) 
        && ($timestamp <= PHP_INT_MAX)
        && ($timestamp >= ~PHP_INT_MAX);
}

This check will only return true if the given $timestamp is a string and consists solely of digits and an optional minus character. The number also has to be within the bit range of an integer (EDIT: actually unneeded as shown here).

var_dump( isValidTimeStamp(1)             ); // false
var_dump( isValidTimeStamp('1')           ); // TRUE
var_dump( isValidTimeStamp('1.0')         ); // false
var_dump( isValidTimeStamp('1.1')         ); // false
var_dump( isValidTimeStamp('0xFF')        ); // false
var_dump( isValidTimeStamp('0123')        ); // false
var_dump( isValidTimeStamp('01090')       ); // false
var_dump( isValidTimeStamp('-1000000')    ); // TRUE
var_dump( isValidTimeStamp('+1000000')    ); // false
var_dump( isValidTimeStamp('2147483648')  ); // false
var_dump( isValidTimeStamp('-2147483649') ); // false

The check for PHP_INT_MAX is to ensure that your string can be used correctly by date and the likes, e.g. it ensures this doesn't happen*:

echo date('Y-m-d', '2147483648');  // 1901-12-13
echo date('Y-m-d', '-2147483649'); // 2038-01-19

On 64bit systems the integer is of course larger than that and the function will no longer return false for "2147483648" and "-2147483649" but for the corresponding larger numbers.

^{(*) Note: I'm not 100% sure, the bit range corresponds with what date can use though}

As a unix timestamp is a integer, use is_int(). However as is_int() doesn't work on strings, we check if it is numeric and its intergal form is the same as its orignal form. Example:

( is_numeric($stamp) && (int)$stamp == $stamp )

I came across the same question and created the following solution for my self, where I don't have to mess with regular expressions or messy if-clauses:

/**
 * @param string $string
 * @return bool
 */
public function isTimestamp($string)
{
    try {
        new DateTime('@' . $string);
    } catch(Exception $e) {
        return false;
    }
    return true;
}

this looks like the way to go:

function is_timestamp($timestamp) {
    if(strtotime(date('d-m-Y H:i:s',$timestamp)) === (int)$timestamp) {
        return $timestamp;
    } else return false;
}

you could also add a is_numeric() check and all sort of other checks.
but this should/could be the basics.

Improved answer to @TD_Nijboer.

This will avoid an exception be thrown if the supplied string is not a time stamp:

function isTimestamp($timestamp) {
    if(ctype_digit($timestamp) && strtotime(date('Y-m-d H:i:s',$timestamp)) === (int)$timestamp) {
        return true;
    } else {
        return false;
    }
}